Membership log me out after second page request

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ASP.NET

membership

Dear All,

I'm facing a wired problem when using asp.net 4.5 membership to manage forms authentication for my users.

The first page i request after login works fine, but when i try to move to another page or even refresh the current page, i get redirected to login page, even though forms timeout was not reached.

I use the following code when users login

if (Membership.ValidateUser(txtEmail.Text, txtPassword.Text))
{
   FormsAuthentication.SetAuthCookie(txtEmail.Text,false);
   Response.Redirect("~/en/pages/");
}

And there is no codes to check if user is logged in inside page or master page, just the traditional web.config settings as follows:

<authorization>                    
   <allow roles="Employee" />
   <deny users="*" />
</authorization>

Thanks in advance.